The present invention relates to a horizontal raster size controller for a monitor, and more particularly, to a horizontal raster size controller in a monitor.
Conventionally, monitors are generally well known in the an to require incrementation of the amplitude of a beam current being applied to a cathode ray tube (CRT) therein in order to brighten screen, resulting in variations in both deflection current and high voltage applied to the cathode ray tube by the regulation characteristics of the flyback transformer. The size of a frame being displayed on a monitor eventually varies owing to the above mentioned variations.
A technique has been developed in which variations of voltage across a primary winding of flyback transformer (FBT) are controllable by a flyback transformer controller which receives a pulse signal having a duty cycle representing a detected variation in the amplitude of a high voltage across a secondary or third winding of the flyback transformer and generated by a high voltage sensor. Namely, when beam current flow applied to an anode of a cathode ray tube increases in amplitude, the size of a frame on the screen of a monitor is expanded, and while the voltage supply across the primary winding of the flyback transformer drops the horizontal size of the frame becomes constant or fixed in size owing to an attenuation of the horizontal beam current.
Such a technique for varying a voltage applied to a flyback transformer as described above causes the characteristics of flyback transformer to substantially change at its high voltage regulation and also to simultaneously vary the voltage level induced at respective terminals of windings on the secondary side of the flyback transformer.
If a voltage applied to one end of a screen resistor and induced across the secondary side of the flyback transformer drops, the brightness of a screen then becomes dimmer. In particular, when voltage applied to a resistor used for contrast control varies, then changes in back raster brightness are visually perceivable. In other words, a frame quality deterioration occurs owing to a significant variation of luminance of a frame having a video signal at a low voltage level.
A high voltage generator including switching transformer for controlling a flyback transformer which outputs a high voltage to an anode of a cathode ray tube is discussed in U.S. Pat. No. 5,278,746 to Tadahiko Matsumoto and entitled High Voltage Generator. The high output voltage is provided to an error amplifier by way of a bleeder resistor and a burlier amplifier. The error amplifier compares the detected high output voltage with a reference voltage such that as the high output voltage drops an error amplifier signal generated by the error amplifier becomes large. The error amplifier signal is input to a comparator for comparison to an integrated inverted horizontal drive signal waveform. The comparator outputs a pulse drive signal whose level rises with the rise of the integrated waveform and falls at a position where the integrated waveform and the error amplifier signal intersect. The pulse drive signal has its width narrowed with the increase in the high output voltage, but has its width widened with the decrease in voltage drop. Accordingly, the duty cycle of the pulse drive signal applied to the switching transformer is varied in response to variations in the detected high output voltage.
An earlier design for a high voltage generator including a flyback transformer which provides a high voltage to an anode of a cathode ray tube is discussed in U.S. Pat. No. 5,317,495 to Toshihiko Furukawa and entitled Stabilized High Voltage Power Supply Circuit. An operational amplifier of a controlling circuit receives the high output voltage fed back through a bleeder resistor and compares the fed back high output voltage with a reference voltage. The controlling circuit includes a switching transistor having its ON/OFF operations controlled by the output of the operational amplifier. The primary coil current of a control tranformer is controlled in accordance with the ON/OFF operations of the switching transistor, which in turn, controls the peak value of the pulse generated at the secondary side of the control transformer for controlling the primary side of the flyback transformer. The high voltage induced by the secondary side of the flyback transformer is therefore controlled in accordance with a variation of the high Output voltage.
A switching mode power supply having overvoltage protection is discussed in U.S. Pat. No. 5,029,269 to Brent Elliott, et al. entitled Delayed Power Supply Overvoltage Shutdown Apparatus. This power supply uses a error detector connected to the secondary side of the power supply transformer for detecting changes in the output voltage applied to a load. The detected error controls the ON/OFF durations, i.e., duty cycle, of a switching transistor connected to the control terminal of the primary winding of the power supply transformer in order to maintain the output voltage at a fixed level.